Method for processing RF signals for receiving and transmission

ABSTRACT

Methodology for processing of RF signals for receiving and transmission, which varies the frequencies of local oscillators, in order to enable the receivers and transmitters to operate with higher performance in wider or multi-bands. bands. Their integrations into ICs are also made easier.

CROSS-REFERENCE TO RELATED APPLICATIONS

This utility patent application claims benefit of and priority toInternational Application, PCT/CN01/01625, filed on Dec. 14, 2001 andpublished as WO 02/56483 A1 on Jul. 18, 2002, which in turn claimspriority to China patent application No. CN 01102802.5 filed on Jan. 12,2001.

FIELD OF THE INVENTION

The present invention relates to a methodology for the processing ofradio frequency (RF) signals during receiving and transmission, andparticularly to such a method for suppressing interference signals in(a) image and other interference signals in receiving and (b)interference to the channel and the receiving sides of the circuit whentransmitting, without using high quality filtering components at RFfrequencies which are difficult to integrate into integrated circuits(ICs).

BACKGROUND OF THE INVENTION

In communication fields, RF frequencies are often used as carriers forsignal receiving and transmission. Using current techniques in thereceiver, modulated RF frequency signals are received and filtered toattenuate interference, especially image signals, before being downconverted to intermediate frequency (IF) frequencies. In thetransmitter, filtering is also needed before the signals aretransmitted, in order to reduce their interferences to communicationchannels and the receiving side of the circuits. In either of thereceiving and transmitting processes, due to the strict signal filteringrequirements, filtering components, such as filters and duplexers,usually cannot be made using normal inductors and capacitors, especiallythose which can be integrated into semiconductor ICs. Instead,components using, for example, dielectric resonators, surface acousticwave elements and the like are needed. While these components workswell, they are bulky in size, expensive, sensitive to manufacturing andassembly tolerances and are difficult to integrate into ICs. This is oneof the major obstacles to integrate a circuit system into a single chip.Zero IF techniques used in receivers solved effectively the imageinterference problem. However, its application has been limited tocertain types of systems, due to issues such as DC offset,sensitivities, and the like, which lower the performance of thereceivers and require demanding compensation and processes.

It is an object of this invention to provide a methodology for RFfrequency signal processing for receiving and transmission that solvethe above problems associated with the current techniques.

BRIEF SUMMARY OF THE INVENTION

According to an embodiment of the invention, in the receiver, a receivedsignal is mixed in mixer 4 with a local oscillator (LO) signal generatedby local oscillator source 5 with varied frequencies. The resultantdesired signal from mixer 4 is selected by IF filter 6 withcorrespondingly varied center frequencies. It can then be mixed again inmixer 7 with a second LO signal generated by local oscillator source 8with correspondingly varied center frequencies and then be selected byfilter 9. In the transmitter, a signal to be transmitted is mixed inmixer 11 with a LO signal generated by local oscillator source 12 withvaried frequencies. The resultant IF signal is selected by filter 13with correspondingly varied center frequencies. It is then further mixedin mixer 14 with a LO signal generated by local oscillator source 15with correspondingly varied frequencies and then filtered by Filter 16.

There are a number of benefits using the process of the invention.Benefits associated with embodiments of the receiver include:

(1) The receiving of the desired signal is effective because it isspread, filter selected, de-spread and filter selected, while image andother interference signals are effectively attenuated, since the otherinterference signals are spread, filter-rejected, spread andfilter-rejected.

(2) Due to the variation of the frequencies of local oscillator source 5and local oscillator source 8, the image interference is not overlappingwith the desired received signal at the 1st and 2nd IF. So filtering ofthe image can be implemented at the IF stages, which can be done withgood performance and lower cost, compared with doing it at front end RFfrequencies. The filters at IF frequencies are also easier to beintegrated into ICs.

(3) Due to (1) and (2) above, the receiver in this invention has relaxedrequirements on the performance of the RF filtering components. Thisleads to (A) it is easier to implement them using inductors andcapacitors, rather than bulkier and more expensive components, such asceramic or SAW components and (B) the receiver can operate at widerbands.

(4) Due to (B) in (3) above, it is easier to use software to set orchange the operating frequency bands by controlling the frequencies ofthe LOs.

(5) Due to (A) in (3) above, it is easier to integrate the wholereceiver, including the front end filtering components, intosemiconductor ICs.

(6) Referring to (3) above, since the requirements for the RF filteringis relaxed, its insertion loss can be lowered at the front end, whichincreases the sensitivity of the receiver.

Benefits associated with embodiments of the transmitter include:

(1) The spurious mixing products of mixer 11 are spread and can then beeffectively rejected, due to the frequency variation of the LO source11.

(2) Due to the frequency variation of the LO source 15, the leakage ofthe LO signal through mixer 14 is spread and has less interference tothe channel and the receiving side.

(3) Modulated signals can be produced with carriers with fixed orhopping frequencies by selecting the ways of the frequency variation ofthe LO source 15.

(4) Due to the spreading of various spurious signals, the filteringrequirement for filter component 16 is lowered. So it is easier toimplement it using inductors and capacitors, making it easier tointegrate the whole transmitter into ICs.

(5) Due to the spreading of various spurious signals, which leads torelaxed filtering requirements for the filtering component 16, thecomponent 16 can be of wider bandwidth. Thus by choosing parameters ofthe varied frequencies of the LOs 12 and 15, which can be programmed,the transmitter can operate in different frequency bands.

(6) Due to the spreading of various spurious signals, which leads torelaxed filtering requirements for the filtering component 16, theinsertion loss of 16 can be reduced, which effectively increases thegain and the power consumption efficiency of the transmitter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of a receiver embodiment based onthe present invention.

FIG. 2 is a functional block diagram of a transmitter embodiment basedon the present invention.

FIG. 3 is a circuit implementation of the receiver in FIG. 1.

FIG. 4 is a circuit implementation of the transmitter in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, the modulated received RF signal is input at PointA to the front end 1, which may have one or more filtering components 2,and/or a low noise amplifier 3. The functions of the front end 1 are toselect and amplify the desired signal, while suppressing unwantedinterferences, especially the image interference. After the processingby front end 1, the signal is fed at point B to the first mixer 4, mixedwith the first LO signal at point C which is generated by LO source 5.The resultant first IF signal is output at point D. Unlike in receiversusing conventional techniques, in this invention, the frequency of theLO signal from LO source 5, f5, is varied as a function of time. Thereare two interesting results of this aspect of the method of the presentinvention. First, an image interference corresponding to a certain valueof the frequency of the first LO signal, f5, becomes a non-imageinterference when that frequency is varied to other values. As a result,except corresponding to that certain value of the frequency of the firstLO signal, f5, the interference above is not overlapped with the desiredsignal at the first IF at point D. So, it can be filtered out at the IFstage. Second, at point D of the first IF stage, both the desired signaland the image signal are spread, whose frequencies vary as functions ofthe variation of the frequency of the first LO signal, f5.

According to an embodiment of the invention, the center frequency of theIF filter 6 is varied and follows the frequencies of the desired signalat point D so that the desired signal is effectively selected and fed tothe input of second mixer 7. The image interference, on the other hand,is further attenuated by filter 6, after its spreading as describedabove. In mixer 7, the desired signal is mixed again with the second LOsignal at point F generated by the second LO source 8. In thisinvention, the frequency of the second LO signal, f8, can be set to varyand synchronize with that of the desired signal at point E. So thedesired signal is both mixed and de-spread by mixer 7 and has fixedfrequencies at point G, and then selected by filter 9 and demodulated bydemodulator 10. The frequency of the second LO signal, f8, can also beset to vary which is not synchronized with that of the desired signal atpoint E. In this case, the output at point G will be of varyingfrequencies. Note that in point E, since the frequencies of the imagesignal vary in general in different direction from that of the desiredsignal (and thus from that of the second LO signal at point F), theimage signal is further spread in mixer 7 and then further attenuated byfilter 9.

It is noted particularly that in this invention, the processing of thedesired signal by mixer 4 and mixer 7 form a spread/de-spread process.As to the image interference, there are two scenarios. In the firstscenario the frequency of the first LO signal, f5, is varied between thefrequency of the desired received signal, fs, and the frequency of theimage interference, fi, at point B, the image interference is spread inmixer 4 and is spread again in mixer 7 (spread-spread). In the secondscenario, if f5 is varied outside of the frequency range between fs andfi at point B, then the image interference is spread by mixer 4 whose fiat point D will have the same distance with fs as at point B. It is thuseasier to filter the image interference out at the IF filter 6 than ifit were performed at the RF section between points A and B. This isbecause while having the same distance between fi and fs, thedistance/fs ratio can be set to be much larger at IF point D than at RFpoint B. So in both scenarios, image interference can be effectivelyattenuated by the receiver in this invention.

Note also that the above-mentioned receiving process is independent ofwhether the desired signal is a spread spectrum signal or not. Thisinvention can be implemented in both cases.

When the desired received signal at point A is a frequency-hoppingsignal, the variation of its frequencies at IF point E is the result ofthe variations of the frequencies of the desired signal and the first LOsignal. Since the frequency of the second LO signal, f8, of LO source 8can be set to synchronize with the varied frequencies of the desiredsignal at IF point E, the desired signal at the output of mixer 7, pointG, can be of fixed frequency. This means that mixer 7 will implementboth mixing and de-spreading of the desired frequency-hopping signal.

Referring to FIG. 3, a circuit implementation of the core functionalblocks from point B to point H of the receiver in FIG. 1 in theinvention is shown. It can be seen from FIG. 3 that the interconnectionsof the functional blocks, namely, first mixer 4, first LO source 5,first IF filter 6, second mixer 7, second LO source 8 and second IFfilter 9, are the same as those in FIG. 1. A circuit implementation foreach of the above blocks is described as follows: Frequency conversionsare performed with mixers 4 and 7. Voltage-controlled oscillators areused as the first and second LO sources 5 and 8, whose outputfrequencies vary as functions of voltages V5 and V8, respectively. Thefirst IF filter 6 is a band pass filter. Its input is at point Dconnected to capacitor 6 a. The other terminal of capacitor 6 a isconnected to the cathode of varactor 6 b, inductor 6 c and capacitor 6d. The anode of varactor 6 b and the other terminal of inductor 6 c aregrounded. The other terminal of capacitor 6 d, point E, gives the outputof filter 6. A controlling voltage, V6, is applied to the cathode ofvaractor 6 b. When V6 varies, the center frequency of filter 6 willvary. The second IF filter 9 is formed with capacitor 9 a and inductor 9b in parallel. One terminal of the parallel circuit is grounded. Theother terminal acts as the input and output points of the filter, i.e.,points G and H.

Referring to FIG. 2, the signals to be transmitted are input at point Iand mixed at mixer 11 with the LO signal at point J generated by LOsource 12. The IF output at point K is selected by filter 13, which alsoattenuates spurious interferences and is further mixed in mixer 14 withthe LO signal at point M generated by LO source 15. Filter 16 selectsthe signal from point N and delivers it to point O for transmission,while rejecting spurious interferences.

In this invention, the frequency of the LO signal at point J is variedas a function of time. The center frequency of filter 13 is also variedin such a way that it follows the variation of the IF signal to betransmitted at point K. So the desired signal is effectively selected,while the spurious interferences are spread due to the variation of thefrequency of the LO signal at point J and are rejected effectively byfilter 13. The frequency of the LO signal of 15 is also varied. If itsynchronizes with the variation of the frequency of the transmittedsignal at point K, the frequency of the output at points N and O isfixed. Otherwise, the transmitter can transmit the signal withfrequencies that are hopping. In either case, the leakage of the LOsignal of 15 to points N and O are spread. So its interference tocommunication channels and the receiving side of the circuit system isreduced.

Referring to FIG. 4, a circuit implementation of the functional blocksof the transmitter in FIG. 2 in the present invention is shown. It canbe seen from FIG. 4 that the interconnections of the functional blocks,namely, first mixer 11, first LO source 12, IF filter 13, second mixer14, second LO source 15 and RF filter 16, are the same as those in FIG.2. A circuit implementation for each of the above blocks is described asfollows: Frequency conversions are performed with mixers 11 and 14.Voltage-controlled oscillators are used as the first and second LOsources 12 and 15, whose output frequencies vary as functions ofvoltages V12 and V15, respectively. IF filter 13 is a band pass filter.The input to IF filter 13 is at point K connected to capacitor 13 a. Theother terminal of capacitor 13 a is connected to the cathode of varactor13 b, inductor 13 c and capacitor 13 d. The anode of varactor 13 b andthe other terminal of inductor 13 c are grounded. The other terminal ofcapacitor 13 d, point L, gives the output of filter 13. A controllingvoltage, V13, is applied to the cathode of varactor 13 b. When V13varies, the center frequency of filter 13 will vary. The RF filter 16 isformed with capacitor 16 a and inductor 16 b in parallel. One terminalof the parallel circuit is grounded. The other terminal of the parallelcircuit acts as the input and output points of the filter, i.e., pointsN and O.

In order to improve even further the performance of receivers based onthe method of this invention, the LO signals may also be generated withmultiple frequency components and the distances between the frequenciesof the component may also be varied. In this case, filters and mixersneed to be designed accordingly to process the multiple components. Thiscan relax further the requirements for the performance of the filteringcomponents at the RF front end, which can make the components moreeasily integrate into ICs.

The functional blocks used in the receiver and transmitter in theinvention, such as mixers, filters, LO sources with varied frequencies,etc., can be implemented using analog circuits. Depending on theoperation frequencies, such functional blocks can also be implementedwith digital circuits. Due to the advancement of digital and devicetechnologies, more and more functional blocks can be realized withdigital circuits.

Accordingly, while this invention has been described with reference toillustrative embodiments, the illustrative embodiments of the inventionare not intended to be construed in a limiting sense. Variousmodifications of the illustrative embodiments will be apparent topersons skilled in the art upon reference to this description. It istherefore contemplated that the appended claims will cover any suchmodification or embodiments as fall within the true scope of theinvention.

1. A method for processing radio frequency (RF) signals for receivingand transmitting, comprising: (a) in a receiver, generating a firstreceive local oscillator (LO) signal by a first receive local oscillatorsource (5), wherein the frequency of the first receive LO signal isfrequency hopped; mixing the first receive LO signal in a Mixer (4) witha received RF signal to produce an intermediate frequency (IF) receivesignal; filtering the IF receive signal, with hopped frequencies, by areceive IF Filter (6) to produce a filtered receive signal; generating asecond receive LO signal by a second receive local oscillator source(8), wherein the frequency of the second receive LO signal is frequencyhopped corresponding to the variation of the frequencies of the filteredreceive signal so that the first receive LO signal and the secondreceive LO signal form a spread and despread pair located in thereceiver; mixing the second receive LO signal in a Mixer (7) with thefiltered receive signal to produce a dehopped receive signal; andfiltering the dehopped receive signal, with a Filter (9) for furtherprocessing; (b) in a transmitter, generating a first transmit LO signalby a first transmit local oscillator source (12), wherein the frequencyof the first transmit LO signal is frequency hopped; mixing the firsttransmit LO signal in a Mixer (11) with a signal to be transmitted toproduce an IF transmit signal; filtering the IF transmit signal, withhopped frequencies, by a transmit IF Filter (13) to produce a filteredtransmit signal; generating a second transmit LO signal by a secondtransmit local oscillator source (15), wherein the frequency of thesecond transmit LO signal is frequency hopped corresponding to thevariation of the frequencies of the filtered transmit signal so that thefirst transmit LO signal and the second transmit LO signal form a spreadand despread pair located in the transmitter; mixing the second transmitLO signal in a Mixer (14) with the filtered transmit signal to produce adehopped transmit signal; and filtering the dehopped transmit signalwith a Filter (16) for further processing.
 2. The method according toclaim 1, wherein the center frequencies of the receive IF Filter (6) areset to vary with the frequency hopping of the IF receive signal, and thecenter frequencies of the transmit IF Filter (13) are set to vary withthe frequency hopping of the IF transmit signal.
 3. The method accordingto claim 1, wherein the variation of the second transmit LO signalgenerated by the second transmit local oscillator source (15) is set insuch ways that the frequency of the dehopped transmit signal is fixed.4. The method according to claim 1, wherein the variation of the secondtransmit LO signal generated by the second transmit local oscillatorsource (15) is set in such ways that the frequency of the dehoppedtransmit signal is varying.
 5. The method according to claim 1, whereinthe processing of the RF signals for receiving and transmitting isimplemented using analog circuits.
 6. The method according to claim 1,wherein the processing of the RF signals for receiving and transmittingis implemented using, partially or fully, digital circuits.
 7. Themethod according to claim 1, wherein the signal outputs of the localoscillator sources (5) and (8) are set to have multiple frequencycomponents.
 8. The method according to claim 7, wherein the distancesbetween frequencies of the multiple frequency components are varying. 9.A method for processing a received radio frequency (RF) signal, in areceiver comprising: receiving a RF signal for processing; generating afrequency hopped first local oscillator (LO) signal; mixing the RFsignal with the frequency hopped first LO signal to obtain anintermediate frequency (IF) signal with frequency hopping; filtering theIF signal to obtain a filtered IF signal; generating a second LO signalhaving a frequency hopping with the frequencies of the filtered IFsignal so that the first LO signal and the second LO signal form aspread and despread pair located in the receiver; mixing the filtered IFsignal with the second LO signal to obtain a mixed signal; and filteringthe mixed signal to obtain an output signal.
 10. A method for processinga signal to be transmitted in a transmitter, comprising: receiving asignal to be transmitted; generating a first frequency hopped localoscillator (LO) signal; mixing the first frequency hopped LO signal withthe signal to be transmitted to obtain an intermediate frequency (IF)signal with frequency hopping; filtering the IF signal to obtain afiltered IF signal; generating a second frequency hopped LO signalhaving a frequency hopping with the frequencies of the filtered IFsignal so that the first LO signal and the second LO signal form aspread and despread pair located in the receiver mixing the filtered IFsignal with the second frequency hopped LO signal to obtain a mixedfiltered IF signal; and filtering the mixed filtered IF signal to obtainan output signal.